The present invention relates to a photosensor system having a photosensor array constituted by two-dimensionally arraying a plurality of photosensors, and a drive control method thereof.
Imaging apparatuses such as electronic still cameras, video cameras, and the like have come to be very widely used. These imaging apparatuses employ a solid-state imaging device, such as a CCD (Charge Coupled Device), which serves as a photoelectric converting device for converting an image of a to-be-photographed subject into an image signal. As well known, the CCD has a structure in which photosensors (light receiving elements) such as photodiodes, or thin film transistors (TFT: Thin Film Transistor) are arranged in a matrix, and the amount of electron-hole pairs (the amount of charge) generated corresponding to the amount of light entering the light receiving section of each sensor is detected by a horizontal scanning circuit and vertical scanning circuit to detect the luminance of radiation.
In a photosensor system using such a CCD, it is necessary to respectively provide scanned photosensors with selective transistors for causing the scanned photosensor to assume a selected state. In place of the combination of the photosensor and the selective transistor, a photosensor (to be referred to as a double-gate photosensor hereinafter) is now being developed, which is formed of a thin film transistor having a so-called double-gate structure and has both a photosensing function and a selecting function.
FIG. 39A is a sectional view showing the structure of a double-gate photosensor 10. FIG. 39B is a circuit diagram showing the equivalent circuit of the double-gate photosensor 10.
The double-gate photosensor 10 comprises a semiconductor thin film 11 formed of amorphous silicon or the like, n+-silicon layers 17 and 18, source and drain electrodes 12 and 13 respectively formed on the n+-silicon layers 17 and 18, a top gate electrode 21 formed above the semiconductor thin film 11 via a block insulating film 14 and upper gate insulating film 15, a protective insulating film 20 provided on the top gate electrode 21, and a bottom gate electrode 22 provided below the semiconductor thin film 11 via a lower gate insulating film 16. The double-gate photosensor 10 is provided on a transparent insulating substrate 19 formed of glass or the like.
In other words, the double-gate photosensor 10 includes an upper MOS transistor comprised of the semiconductor thin film 11, source electrode 12, drain electrode 13, and top gate electrode 21, and a lower MOS transistor comprised of the semiconductor thin film 11, source electrode 12, drain electrode 13, and bottom gate electrode 22. As shown in the equivalent circuit of FIG. 31B, the double-gate photosensor 10 is considered to include two MOS transistors having a common channel region formed of the semiconductor thin film 11, TG (Top Gate terminal), BG (Bottom Gate terminal), S (Source terminal), and D (drain Terminal).
The protective insulating film 20, top gate electrode 21, upper gate insulating film 15, block insulating film 14, and lower gate insulating film 16 are all formed of a material having a high transmittance of visible light for activating the semiconductor thin film 11. Light entering the sensor from the top gate electrode 21 side passes through the top gate electrode 21, upper gate insulating film 15, and block insulating film 14, and then enters the semiconductor thin film 11, thereby generating and accumulating charges (positive holes) in the channel region.
FIG. 40 is a schematic view showing a photosensor system constituted by two-dimensionally arraying double-gate photosensors 10. As shown in FIG. 40, the photosensor system comprises a sensor array 100 that is constituted of a large number of double-gate photosensors 10 arranged in an n×m matrix, top and bottom gate lines 101 and 102 that respectively connect the top gate terminals TG and bottom gate terminals BG of the double-gate photosensors 10 in a row direction, top and bottom gate drivers 111 and 112 respectively connected to the top and bottom gate lines 101 and 102, data lines 103 that respectively connect the drain terminals D of the double-gate photosensors 10 in a column direction, and an output circuit section 113 connected to the data lines 103.
In FIG. 40, φtg and φbg represent control signals for generating a reset pulse φTi and readout pulse φBi, respectively, which will be described later, and φpg represents a pre-charge pulse for controlling the timing at which a pre-charge voltage Vpg is applied.
In the above-described structure, as described later, the photosensing function is realized by applying a predetermined voltage from the top gate driver 111 to the top gate terminals TG, while the readout function is realized by applying a predetermined voltage from the bottom gate driver 112 to the bottom gate terminals BG, then sending the output voltage of the photosensors 10 to the output circuit section 113 via the data lines 103, and outputting serial data Vout.
FIGS. 41A to 41D are timing charts showing a method of controlling the photosensor system, and showing a detecting period (i-th row processing cycle) in the i-th row of the sensor array 100. First, a high-level pulse voltage (reset pulse; e.g., Vtg=+15V) φTi shown in FIG. 41A is applied to the top gate line 101 of the i-th row, and during a reset period Treset, reset operation for discharging the double-gate photosensors 10 of the i-th row is executed.
Subsequently, a bias voltage φTi of low level (e.g., Vtg=−15V) is applied to the top gate line 101 of the i-th row, thereby finishing the reset period Treset and starting a charge accumulating period Ta in which the channel region is charged. During the charge accumulating period Ta, charges (positive holes) corresponding to the amount of light entering each sensor from the top gate electrode side are accumulated in the channel region.
Then, a pre-charge pulse φpg shown in FIG. 41C with a pre-charge voltage Vpg is applied to the data lines 103 during the charge accumulating period Ta, and after a pre-charge period Tprch for making the drain electrodes 13 keep a charge, a bias voltage (readout pulse φBi) of high level (e.g., Vbg=+10V) shown in FIG. 41B is applied to the bottom gate line 102 of the i-th row. At this time, the double-gate photosensors 10 of the i-th row are turned on to start a readout period Tread.
During the readout period Tread, the charges accumulated in the channel region serve to moderate a low-level voltage (e.g., Vtg=−15V) which has an opposite polarity of charges accumulated in the channel region and is applied to each top gate terminal TG. Therefore, an n-type channel is formed by the voltage Vbg at each bottom gate terminal BG, the voltage VD at the data lines 103 gradually reduces in accordance with the drain current with lapse of time after the pre-charge voltage Vpg is applied. More specifically, the tendency of change in the voltage VD at the data lines 103 depends upon the amount of received light in the case where the charge accumulating period Ta is constant. As shown in FIG. 41D, the voltage VD tends to gradually reduce when the incident light is dark, i.e., a small amount of light is received, and hence only small charges are accumulated, whereas the voltage VD tends to suddenly reduce when the incident light is bright, i.e., a large amount of light is received, and hence large charges are accumulated. From this, it is understood that the amount of radiation can be calculated by detecting the voltage VD at the data lines 103 a predetermined period after the start of the readout period Tread, or by detecting a period required until the voltage VD reaches a predetermined threshold voltage. An image reading sensitivity corresponds to the charge accumulating period Ta. Assuming that the amount of light is constant, the amount of charge accumulated increases and the image reading sensitivity enhanced in accordance with an increase in the charge accumulating period Ta. Likewise, the amount of charge accumulated decreases and the image reading sensitivity degraded in accordance with a decrease in the charge accumulating period Ta.
Image reading is performed by sequentially executing the above-described drive-control for each line of the sensor array 100, by executing the control for each line in a parallel manner at different timings at which the driving pulses do not overlap.
Although the case of using the double-gate photosensor as a photosensor has been described above, even a photosensor system using a photodiode or phototransistor as a photosensor has operation steps: reset operation→charge accumulating operation→pre-charge operation→reading operation, and uses a similar drive sequence. The conventional photosensor system as above has the following problems.
(1) To read a subject image in various use environments in a photosensor system using the above-described photosensor, the image reading sensitivity (charge accumulating period) must be properly set. The proper image reading sensitivity changes depending on changes in ambient conditions such as the illuminance of external light in a use environment, and also changes when the characteristics of the photosensor change. In the prior art, therefore, a circuit for detecting the illuminance of external light must be additionally arranged. Alternatively, a subject image is read at different image reading sensitivities before the start of normal reading operation of that subject image, and an optimal image reading sensitivity must be determined on the basis of a read result. However, a reading sensitivity setting method of unconditionally and automatically setting a proper image reading sensitivity based on a read result for every image reading sensitivity that is obtained by operations executed at different image reading sensitivities, has not been developed yet.
(2) If a foreign substance attaches to the sensing surface of a photosensor or a defect is generated in a photosensor element in setting the reading sensitivity based on the result of the sensitivity-adjusting reading operation, and a read result obtained for each image reading sensitivity obtained in the reading operation is directly used, an abnormal value is contained in the read result, leading to failure to set a proper image reading sensitivity and to inhibit accurate reading operation of a subject image. For example, when this photosensor system is applied to a fingerprint reading apparatus, the apparatus may malfunction in fingerprint recognition processing.